On the one hand: Junkrat being immune to his own bombs but Pharah being vulnerable to hers makes sense from a game play perspective, as Junkrat does more close quarters fighting and Pharah is designed for long range
On the other hand: It make no sense that Pharah’s advanced battle armor provides less protection than Junkrat’s crappy jorts
On the other other hand: Junkrat having blown himself up so many times his immune system adapted to it is the best possible thing to happen in Overwatch’s lore